Left-handers Have A Larger Brain Part, Study Finds

WASHINGTON — Left-handed people may not be superior to right-handers, but southpaws may rejoice that part of their brain is bigger, the report of a new study in Canada said Thursday.

Post-mortem examination of 42 people showed that a bundle of nerve fibers called the corpus callosum, which connects the two halves of the brain, was larger in those who tended to use their left hands for many tasks.

The size disparity may help explain other differences between lefties and right-handers in areas such as how they process information, said Dr. Sandra Witelson, a professor of psychiatry at McMaster University in Hamilton, Ontario.

''Certainly bigger is not necessarily better,'' said Witelson, writer of the report in the journal Science.

''What I think this is saying is that right-handers and left-handers do have a different brain anatomy,'' Witelson said.

The corpus callosum connects the right and left halves of the brain. Each half has different duties, but the corpus callosum makes it possible for the halves to communicate and, to some extent, to share jobs.

The left hemisphere is responsible for controlling the right side of the body, while the right half controls the left side. Beyond that, the left hemisphere handles language, mathematics and guidance of voluntary movement. The right handles spatial and musical perception, among other functions.

Earlier studies have shown that people who use their left hands often have less division of labor between the two halves of the brain, Witelson said. For instance, lefties handle language more on both sides of the brain.

Lefties also recover more quickly from injuries to one side of the brain, she said.

Because the corpus callosum is bigger in lefties, ''it becomes a very tantalizing speculation'' that this structure may be responsible for the difference, she said.

''It sort of raises the hypothesis that the larger track is the one that allows for greater communication between hemispheres,'' she said.

It is not certain whether the larger corpus callosum has more fibers or simply more space between fibers.

Witelson pointed out that being right-handed or left-handed is not necessarily defined by which hand a person uses to write.

About 65 percent of the population is completely right-handed. That means their right hands dominate in two-handed tasks, and they use their right hands in one-handed tasks, such as brushing their teeth or using scissors.

Thirty-five percent of people are not completely right-handed. This means they use their left hands for some tasks, which may or may not include writing.